The present invention relates to the use of high frequency microwave (HFMW) radiation, preferably in the form of a beam, to make hybrid metal ceramic composites and to the resultant products. It relates particularly to the combination of preceramic polymer infiltration with high frequency microwave pyrolysis referred to as xe2x80x9cPIMPxe2x80x9d processing.
Ceramic composites, such as continuous fiber-reinforced ceramic composites (CFCCs), are inherently wear and erosion resistant, retain strength at higher temperatures and are lighter in weight than competing metals. Many applications, ranging from turbine engines and rocket nozzles to gun barrels require repeated or extended exposure to aggressive gaseous species at high temperature and pressure to form such products.
The composites formed have to be combined with other materials, such as metallic elements, and parts to form the final product, such as a turbine engine. Ceramics, including ceramic composites, are inherently brittle, while metals are inherently ductile. These inherent properties render the attachment of ceramics and ceramic composites to metal structures problematic.
Development of effective methods of processing of CFCCs has been a subject of intense investigation for over 15 years, and several approaches with potential for industrial utilization have been identified. While chemical vapor infiltration (CVI) technology is currently viewed as the industry leader, it is a slow, complex process with many inherent difficulties, including a corrosive gas environment, a high cost for process tooling and a substantial amount (15-25%) of residual porosity. Although some of these issues have been mitigated by new CVI methods, few metals can tolerate the highly corrosive CVI atmosphere, and those that can (e.g., tungsten alloys) have very high specific gravities, adding substantial weight to the structure.
This is particularly true of products in which the metal is in the form of tubes. Currently, three basic methods are used to produce actively cooled CFCC components: i) heavy, refractory metal tubes are co-processed with the CFCC; ii) metal tubes are brazed to the CFCC; or iii) a very dense matrix CFCC is processed with cooling passages into which a metal liner may or may not be inserted. All three approaches suffer from substantial shortcomings.
In addition to new methods of producing fiber-reinforced ceramics, simple, rapid and reliable methods of attaching the CFCC to the metal support structure are needed. Although fiber-reinforced ceramics are much less brittle than their monolithic counterparts, they are not ductile like metal components. In addition, dense ceramics and ceramic composites are difficult to machine. For these and other reasons, conventional attachment strategies are inadequate. The attachment issue could be simplified greatly if metallic features, such as attachment lugs, could be co-processed as an integral part of the ceramic component.
Polymer infiltration/pyrolysis (PIP) processing is a new method of manufacturing CFCCs that is a simple extension of the traditional methods used to manufacture carbon-carbon and polymer-matrix composites. PIP processing is inherently compatible with the intelligent manufacturing techniques currently under development for polymer matrix composites, and has been shown to yield CFCCs with properties equal or superior to those produced by other methods. It is currently practiced by a number of firms using conventional pyrolysis methods. However, because of the change in density associated with the conversion of the matrix precursor to the finished ceramic, repeated infiltration/firing cycles are required to produce a dense finished ceramic product. Economic models developed as part of the DARPA Low Cost Ceramic Composites (LC3) Program indicate that more than 30% of the cost of a specific CFCC part is derived from the time consumed during pyrolysis.
Further, it has not been possible to utilize these prior procedures to include metallic elements as an integrally formed part of the finished ceramic product. This is due to the fact that the CVI and conventional PIP processes require such high temperatures to convert the preceramic to the finished ceramic product that metallic elements processed therewith are adversely affected.
Also in the case of CVI processing, the pyrolysis atmosphere contains corrosive gases, such as HF and HCl, which corrode all but the most corrosion resistant metal elements.
The present invention overcomes the problems of the prior. art and provides for the co-processing of metallic elements and fiber performs infiltrated with a ceramic precursor to form hybrid metal/ceramic composite products without degradation of the metallic element, regardless of its composition.
Briefly stated, the present invention comprises the method of making a ceramic composite containing at least one metal element comprising associating a polymeric ceramic precursor with at least one metallic element therewith to form a preceramic composite, and subjecting said composite to high frequency microwave radiation, preferably in the form of a beam, until the polymeric preceramic precursor is converted into a ceramic. Preferably, the polymeric ceramic precursor is infiltrated in a fiber reinforcement.
The invention also comprises the resultant products are ceramic composites having metallic elements integrally formed therewith as hereinafter described.
In this process, the fiber acts as a susceptor, and becomes hot when exposed to HFMW radiation. The hot susceptor in turn induces the conversion of the preceramic polymer to ceramic. Other material additives, such as powders, flake, whiskers, and chopped fibers, can be employed as susceptors if they are found to posses the required dielectric properties (e.g., dielectric constant, loss tangent, etc. as known to those skilled in this art). Silicon carbide and certain forms of carbon have been found to function adequately as susceptors; however, any material with the appropriate dielectric chemical and physical properties as, again, known to those skilled in this art can be used, may be employed.